This topic is likely more suited to be in the "Beyond Dreaming" section, where I'd guess similar topics have appeared before, as I don't think this is necessarily a novel idea - the specifics probably differ in some way.

I could ask a counter-question to your topic title; What if you and everything else are a figment of my imagination (or vice-versa)?

Growing up, I didn't instinctively know or understand that other people felt the same (or similar) things as I did and in many respects I did see the world around me as a kind of construct that was existing only for me - regardless of that existence being positive, negative or something in-between as I lived it. By that point, only a few years into my conscious life, I was the only thing I understood to be "conscious" and sleeping had always been a weird phenomenon for me as a child, especially as other people in my life would often talk about their dreams and daydreams, things that I barely recognised in myself at the time.

Although originally I had those feelings about reality, I can't really say that I lived accordingly for any considerable length of time; if I had, I would have treated everyone else as some psychopaths and so on might do, with very little regard for other things and people. At one point, after a minor and otherwise insignificant event, I chose to discard my earlier view of reality. If I hadn't, I wouldn't have been able to compromise properly with others, in order to live life itself.

In what you're talking about, or even in the way I originally conceived of reality as a child, there is no more truth than in believing something else, because anything can feel true and even proven, under the right circumstances and context.

In addition, regarding the context you put forward, it assumes that the outside "reality" of this one would work in some comparable way to what we know. Any number of assumptions could be made about an unobservable "outer" reality.

Truly, I can't say that your hypothesis is false or that mine was ever true; I can only truly say that I can't believe in them. You'll be damned if you believe someone or something else controls the mind of those who aren't with you and you'll be damned if you don't believe it, all the same, because either way, their behaviour will likely remain the way it is, or be reinforced in the face of your beliefs being made known.